Monitoring devices to determine brake lining wear have already been proposed. Thus it is known from vehicle manufacture to include a conductor in the brake lining material so that it is ground through with corresponding wear of the brake lining material due to braking. Once this conductor is ground through, the resistance is infinite when the brake is raised, i.e. when the brake lining does not contact the brake disk or the brake drum, and a certain finite resistance exists when the brake is in use, i.e. when the brake lining contacts the brake disk or the brake drum. In the presence of a predetermined wear condition, this monitoring device permits to generate a warning signal which informs the user of an impending brake lining change. However, this monitoring device is unable to detect a condition in which the brake lining has become nonfunctional due to contamination.
This has caused big problems, particularly in elevators. They typically have an electromagnetically activated brake as a component of the motor unit, which is used to hold the car when it stands still at a landing. These brakes are typically drum brakes, where the brake linings act on a brake drum from the outside and are pulled into the braking condition by a spring. A solenoid is used to raise the brake. The car can move when the brake is raised. This brake is extremely important because the electric motors or any interposed gears cannot exert sufficient braking action for a standstill. During a brake failure the car moves either up or down depending on whether the counterweight is lighter or heavier than the car plus its load. Since the car door and the shaft door are typically open in this condition, it can be a highly dangerous situation for the passengers in the car. In addition there is the danger of a crash when the car suddenly moves due to load changes during loading and unloading. Such situations have already occurred in the past, but fortunately without serious consequences in most cases. The cause of the brake failure in the majority of cases was brake linings soaked with lubricants which had leaked from the motor unit.
Unlike in motor vehicles where the driver can detect a gradual failure of the braking effect by a “spongy” feeling, the situation with elevators is that there is typically a sudden failure unless the elevator installation has been serviced by chance, when a beginning contamination of the brake would be detected.
It is therefore the objective of this invention to provide a method of monitoring the functionability of a brake lining, whereby in particular an impending brake failure due to contamination can be detected in time.